The invention described herein relates generally to OFDM wireless communications, and more particularly to an OFDM transmitter that accommodates variably sized input data blocks.
Orthogonal Frequency Division Multiplexing (OFDM) is a digital multi-carrier modulation technique that uses a plurality of closely-spaced orthogonal subcarrier frequencies to carry data. OFDM operates by dividing a spectrum of transmission data into a multiplicity of narrowband sub-channels with a specific spacing termed “orthogonal spacing,” where a fraction of the total data rate specified for the transmission data is modulated onto each sub-channel with a conventional modulation scheme (e.g., quadrature amplitude modulation). With OFDM, different subcarriers are allocated to different users, which allows several users to share the available bandwidth. OFDM is deployed or planned for a variety of wireless systems, including IEEE 802.16 (WiMAX), some IEEE 802.11a/g wireless LANs (Wi-Fi), IEEE 802.20 Mobile Broadband Wireless Access (MBWA), and the like.
A practical implementation of an OFDM transmitter presents data symbol values associated with different subcarrier frequencies of an input data block to different frequency-domain inputs of an Inverse Discrete Fourier Transform (IDFT) processor to generate a time-domain representation of the desired transmission waveform. Subsequently, the digital time-domain representation is converted to an analog stream of modulated symbols. After amplification, the analog symbol stream is wirelessly transmitted to a remote device.
OFDM transmitters are typically designed for a fixed data rate and/or bandwidth. However, wireless communication services provided by OFDM transmitters often involve highly asymmetrical data flow associated with asymmetrical data rates. For example, with mobile internet browsing mouse clicks are transmitted on the uplink while images or other voluminous data are transmitted on the downlink. In another example, a camera-phone may be used to take and email a picture. In this case, voluminous data is transmitted on the uplink while only acknowledgements are transmitted on the downlink. Due to the potential for asymmetrical data flow, a conventional OFDM transmitter may be designed with a fixed data rate and/or bandwidth that accomodates a worst-case scenario, e.g., the scenario having the highest data rate and requiring the widest bandwidth. However, such fixed data rate and/or bandwidth transmitters typically do not efficiently use the available spectrum and power. Ideally, OFDM systems would have the ability to operate at less than the maximum bandwidth by using only a subset of subcarrier frequencies, where the unused spectral slots are fed zero symbol values. However, this approach generally leads to interference with neighboring signals. In particular, while the energy transmitted in unused slots typically drops to the digital noise floor, the non-zero signal energy in the unused slots may still be large enough to interfere with neighboring signals. Thus, there remains a need for an OFDM transmitter that efficiently uses the available spectrum and power while minimizing interference caused by unwanted spectral components in unused spectral slots.